Two-stage pulping process



May 26, 1953 Filed June 15, 1948 (500A NEUTRALIZED/ JODA CHARGED) Z 2 Sheets-Sheet 2 Cmzva A l-O Hduuzs AT MAX. Tmnmo'c) CURVE B 0-5 HOURS AT MAX. TEMP. (I70'C) 0 0 7 8 9 IO N IZ 500A CHARGED Na OH on OVEN DRYWOOD) I W M W-MWW "persed ire-18 20'mls.-=-1'0%sulphuric am '50 'mlsf of" tenth'norma1 patassium permanei 55 ganate solution "Thei -reaction is held eat-20 C. and carried out With-constant stirring for exactly ganatei inls; or"

Patentecl May 26, 1953 awe-erase runrmfirliocnss Arthur-Raymond sloman, Burnie, Tasmania,

Australia assignor Pape'r Mill's lzimit'ed,

to Asscciated Pu-lp and MelbournaWictoriapAus- .trialiai, a corporation of Victoria AnplicationJune 15, 1948, Seria1NQV-;3.3,Q35

. .111 Australia J pl 1 .Giaim.

This invention relates to improvements in the production of p-aperpulpa-and. refers especially to mprovements in the "cooking of wood *by the alkalineprocesses for theproduction "of; it paper I plup. The-invention "applicable'ta' the "soda.

-or lother cellulose-bearing rawimiaterials has been to-xcook .these...materials @in. a;;single-stage cook, theiactiver-rchemical:being.added either at the 'scemmencementiof the-ficook 'oiz by injection underipressure during the courseuofrthe cook. Therev have :been developments towards multi stage alkaline cooking of cellulosic materials for the production of pulps; but theseprocesses have not been entirely satisfactory, they have not efiected the d'esired'econorniesi in operation, in

some instances they require cumbersome operating procedures, and in most'cases they:.have-;re-

quired the addition of some new caustic-sddain each stage of the process.

I have discovered that it is possible to produce easy-bleaching pulps from'wood-bythe use of a two-stage 'prooessz in which? the; 'eiiluent .liquor from thei-second stages 1536113811 in the-first stages of snbsequent cooksswithoutxtheaddition of any substantiahquantity;or.new activeichemical- (e. g.

. augtilcirggdi-l, as exilicpio-rfir'r in;. .the firststagec atmentsrir :Bys. thisfimeans. considerable savings ban-be effected instheramount oixactivechemical re'quired to'rproduce pulps withssthe Samar-char- I acteri'stics.

" *In'this specificatiorrtherternrflfiwhite liquor is. 'usedto refer-to aiso'lutio-n .containingirecovered ihali and/or i1 eshsalkaliin:suchconcentration to'render the 'so'lution suitable-for.the alkaline pulping of :wooa: and: the :term' .black liquori-i is used to refer to. the -"liquor :discharged. from .then digester after thetalkaline cooking and; delignifi- 1 cation of wood and which contains iboth airee alkalianci alkalhcombined'withrorganic materials dissolved frorn the'wood.

In this specification the Theequivalent (if 12000! gms. O. D; pulp-is dismlsfi of":- distilled water et C. d is added iollowedby five minutes after .the' -addition' of -the perman- At" the en-d of"@the ractionrperiod .Permanganate 1111111 45 Pomadem's smithzL. ber ofa-pulp is sdefinedias thet number of millilitres of decino-rmalpctassi-umwpermanganate reducedby LUOO g'msi -oven==dry 'pulp ifs-five? minutes when the pulp instreated ivitil -the=perman-- ganate according to -.thertest conditions *SQ'BifiGdQ? below:

tenth normal ferrous ammcniums'isulei 6,0; cocky.

(Cl:.i.92++13) i 1 phate-(ecidirledwith -mls. -concentrated sulphuric-aeid per litreof solution is added to stop i the reaction. I 1 ==lhe-excess I of ferrous a-ammonium sulphate then-iback titrated with tenth-normal v 5-potassiumvpermanganate. Thenumberoi milli- -=-litres c t-potassium permanganate solution .re-

quired for the back-titration is the permanganate Where the permanganate number of the pulp 14) exceeds 2Ci when carried out according to the above method the volume of potassium permanctganate solution, sulphuric acid, water and ferrous ammonium sulphate are increased until the ratio of volume of permanganate added to volume of permanganate reduced is not less than 2.5. 'lhe ratio-of volume of permanganate added, to volume of 163% sulphuric acid, to volume of water 15 is heldoonstant at 522218 when such changes in ithermethod are necessary.

It l have found experimentally that the black 4- =.liomor obtained from normal single stage alkaline ,v p n which caustic soda, either alone or in 'onjunetion with sodium carbonate or sodium ---sulphide; is the active chemical) contains sub- 25-'stantialeuantities of unneutralized caustic soda e rand othercaustic: alkalinity In fact, it hasbeen ascertained that..in-,-.cooking Eucalypt s yi qm re bycthe spdaprocessin ermal'si slefiiasei with a soda. charge gofeforr-example, 2.4%, NaGl-I l iqahNanl-ll on] the O2 D.'..W.0 0. d is neutralizedby therwoodnscwith. radiate ayco nifen;, com- N a-OH is neutralized with ,as soda charge 033,475

: 5 NaOH/G; D. -.;wcod. .;-:-Simi-1a1f results with other woodsare listedsbelow ,ees.

In these: experiments QtheQNaOI-I remaining in :tl'iegblaclrili uorr;..wasr.determined potentiometrially.

. gIheruse of sdlif'erent. soda: char es with the. one cod, providingtheisoclacharge is in excess of tie -trivalent to..the amount shown to heneualiaed the (1001;; does 1 not ;.materially 5: affect witheg-iampunt of causticvsodaneutralized in .-.the whenpulps eithesamebleach requirement (indicated by the permanganate number) are produced from the one wood. Over the ranges of soda charge shown in Table 1 for each wood, the amount of soda neutralized was constant at the amount shown in the table for that wood. In all cases the pulps produced had permanganate numbers close to 15.

It appears possible that the main function of the additional caustic soda used over the amount neutralized is to provide a greater mass of causdepress the rate of lignin removal, or, in the case of cooks conducted for a given time, to result in the production of pulps having higher permanganate numbers.

A series of experiments was carried out by cooking a sample of Eucalyptus gigantea with increasing amounts of black liquor, and maintaining constant the charge of caustic soda added by way of. white liquor. All the cooks were charged with 20% NaOH/O. D. wood added as tic soda in order to increase the speed of the dewhite liquor, and the liquor ratio (1. e. mls. liqlignification process, or alternatively that the uor/gms. O. D. wood) was made up to 5:1 by physico-ohemical condition of the wood is afiectthe addition of either black liquor or water or ed in a way which improves the rate of difiusion both, and in one case by the addition of more of the sodium lig-nin complex from the fibers highly concentrated black liquor. The cooks and intercellular zones. Thus in pulp mills where were heated to 170 C. over a period of two hours single stage alkaline cooking is practised, suband held at this temperature for a period of two stantial quantities of unneutralized alkali are hours. The cooks were then shock-cooled by returned to the alkali recovery system. In this being placed in cold water. The conditions used system the caustic soda is converted to sodium and the results obtained are shown in the table carbonate and then requires to be recausticised. below:

TABLE 2 iquor Soda neusolids Permm Pulp $3 tralizcd Cook addedin {mate yield (unsoda during No Liquor liquor, of bleached), added cook,

' gms./100 pulp exigent or; gms NpggegtD gms. .woo a 0.1). NBOH wood wood 1 121 mls. white liquor containing Ni] 14.5 52.5 20.0 13.0

20.0 gms. NaOH and 360 mls. distilled water. 2 121 mls. white liquor (as above), 1035.... 10.3. 53.1 25.6 13.0

250 mls. black liquor, 110 mls. distilled water. 3 121 mls. white liquor (as above), 320 approx. 51. approx.60 37.2 13.2

350 mls. concentrated black liquor containing 160 gmsblack liquor solids obtained by evaporating 770 mls. of black liquor (as above) to 350 mls.

thereby greatly increasing cost without doing more useful work than increasing reaction rate.

Now, the black liquor thus normally returned to the recovery system is not useful alone for further single stage cooking owing, firstly, to the relatively low concentration of free alkali, and secondly, to the presence of high concentrations of wood solids (e. g. lignin, hemi-celluloses and other organic materials) I have found, however, that the black liquor can be used as a source of soda for neutralizing much of the natural acidity in wood and thereby reducing the quantity of new caustic soda required in the pulping process. 1

From my experiments, it appears that black liquor can only be used effectively for this purpose if the cook is carried out in two stages with the addition of black liquor alone or together with relatively small amounts of other alkali in the first stage.

In normal single-stage cooking, it is known to employ black liquor from previous cooks as liquor make-up to cover the charge of wood. It is recognised that the free or loosely combined caustic soda in this liquor when used according to normal practice cannot be regarded as behaving as part of the active chemical charged to the cook, either for the purpose of neutralising wood acids or for effecting delignification, in that the cooking time required to produce a pulp of given permanganate number is not reduced by increas- The black liquor used in the above experiments contained 207.5 g. p. l. of total solids and 22.4 g. p. l. caustic alkalinity expressed as NaOH.

It can be seen from Table 2 that the addition of black liquor tended to result in the production of pulps with higher permanganate numbers, although the amount of soda neutralized was substantially constant This is probably due to the presence of substantial amounts of wood solids (lignin hemicelluloses and the like) in solution in the black liquor, the eiiect of which is to more than counteract the increase in rate of delignification which would be expected to follow from the increase in active alkali concentration obtained by the addition of black liquor.

The presence of wood solids in solution appears to have the effect of preventing diffusion and/or dissolution of the material of the wood. even though this material has reacted chemically with the normal amount of caustic soda.

I have also discovered that the neutralization of natural acidity in wood can be carried out with solutions of relatively low alkali concentration, and that this process does not appear to be adversely affected by the presence of wood solids in the liquor. It is possible to neutralize up to about 10% NaOH on O. D. wood (Eucalyptus gigantea) and probably higher amounts in the case of woods having a higher content of acidic materials, using liquors weak in alkali (i. e. in concentrations less than, say, 40 gms. per litre with a liquor ratio of, say, 3.5 to 1) and carrying substantial quantities of wood solids.

Ithas also been found that if the cooking or digesting treatment of wood is conducted in two .cooking. a I:

The process is carried out in a closed vessel,

sin le stage cook employing 24% NaOH on O. D.

I, 1 5rd,; ,1; a Hm A; 5.1.7 n? v 1 based Qnthese tidings h r ;appreciable savtcooking wood. and l kemater als can be effected.

An important feature of the invention resides in,using,;,.the; "liquor eiromvthe .-s econ i stage/of a two-stage cooking treatment toL-iprovidetthe alkali necessary for neutralizingitheynatural acidity 30* of woocltima-subsequent first stage treatment a s e, invention broadly includes a ...two-stage alkal netprooess. for the. production. of paper pulp lrom;,;wood which, comprises subjecting-a wood 9h mjQfjla firstestage digestion treatment --to llbstantially neutralize its mat-m1 acidity; by coolsing-thewoodsina closed :vessel'in black liquor at..atemperature between z,150- C-;. and 180 the. caustic .alkalinitytofi th blackliquonuseglin said first-stagetreatment being between 5% and 1 16% Expressed as NaOI-Inon seven-dry wood) andu=from't80'% t-to about- 0.% of said alkalinity being eneutralized ,in said treatment, separating the resultinglsubstantial-ly exhausted liquorfrom the wood charge;.and:- then subjecting- -the wood charge to aosecond-stage.delignification treatment; by cooking the wood--char -ina-a closed Vessel in a digestion- -liquor comprising white liquor at a temperatu re between 150 C. and. 180? C. to produce a pulp a of the digestion liquor uslegl thesec tage treatment'be'ing' such that the causticalkal mty of the black honor-separated 'frornthewood ch rge after the saidsecond-stage treatment is less than RP/Q lbs. NaOI-I per gallon where R. alkali-.tcharge pen-.cent. onovem dry wood) atwhich 80%.olthe alkaliisneutralized byqwoodbin ,ayfirst-stage treatment,

P=ooncentration lot-"caustic ,alkalinity in w the I lblacle liquor leaving.-the seconglastage divided hyttheooncentration of caustic alkalinity in the -z sliquor.after dilution,.l,m.n i

Q yolumei-ofi iblackcliquor charged to first-stage treatment (in gallons per 1110 lbs. oven-dry h woodlut mI-The lmethod-tofcalculating; by-peans oi the above mentioned formula,-th e maximum quantity of caustic alkalinity to'beusedin-the secondstage treatment w ill be more fully explained later A g rsH fiifii -z m J fr- 2. m A M" Black liquor may also be added as m k up" liq "the sseeaa ta et ea msottam k the; uired volume, as in normal single stage m y'v. t remtthe 20) U e caustic alkalinity .s .i

5O havediscovered;fthatst e iqu Pdlfiqhfiged =5 he a l 1 t assesses. el??- hours at maxim erature.

T ethco i s i1 i um emp ra ar la-11 9f C-iSZ Q L i tween; .0 and 3.0 hours, and is preferably about labours-j 1 w h the Woods in use; a d in H k tdesi ble that the mo nt.- Of ca t a k n be. between; 0% andt'la z 0 a required. to; neutralize the, n tural v ac whoa when. theater s c pkedina f sl 2 zeo k tolc velpu i o the. l chab l txie l i td Thus with Ficus madiatig, a, wood reguiringa r ively z hi h a kali charse der nb ina1 o ateconditions,the'amount o aust c, a kal nit 40 introduced by way of black liquor shoulglpreferrutb y; be between- %!..1anc1\ 43%, fixl 'rested. 1 age: on;O,.D. wood. -WithEycalyptus gigantca relativefigureswould bet to 110%., t;

alkali addition inithe iii-sta d second stage "treatments is also controlled so that at least 80% of the caustic alkalinitybf the black liquor used eisyneu rali ed iu the:first s a flireatmentt -lmeth cl ofscontrolinece s rato an In bes moreiu y a er in this. pecifica i n-H;

from the first stage treatment is particularly suitaable fori the recoveryn-oi. the lignin and other organicmaterialsttherefrom, v: A. ifurthe i fe ture-ofthe; 12 9 .1. re ides, i

n h etr t t atuby means this p oces thequ pu aq p pm y be in reaseda n., ll, iconssu p o :Inp her. w s inam ll h v n 2 .t adas e p e aandl ca sti i w n qaaaq t and-,ifsadequate; .7 eest q i; ca ci il is aya ail '0 ,sit, isipossible to substantially .increa ethe,outppt. ;-.o i pulp tr mth m ll-L 15 an ic t n Qiih process involves an i11cre'asein ;the 10 of yvood solids to inorganic materials in the black liquor passed to the recovery system, and consequently a incre se e enerat aa n g st ae capa thenulaput ut i nerea e iti ahlap t tt ah ask li eorlvisj-ava it is i is l? e ifer ltd imit tin da IQQOYQ p nt in t latter are high 'anii'labour flimcqltiesQS OQlQ QQW h its a mfi qaa s 9 oreo er ani r a e weo .oves the burnin qualitiesoffthe vention, pulp produced by the two-stage process of this invention operating on a given time cycle possesses certain improved physical characteristics compared with pulp produced by the normal single stage process operating on the same time cycle.

A still further feature resides in the fact that in the process of this invention variations in cooking time required to give pulp of constant permanganate number (i. e. bleachability) consequent upon uncontrollable changes in alkali charge (per cent alkali on O. D. wood) are much less and affect the cooking cycle by a smaller time than is the case with normal single stage cooking.

I have ascertained by experiment that in soda pulping the graph of soda charge (per cent NaOH on O. D. wood) against the reciprocal of the time of cooking to produce at any one maximum temperature pulps of given permanganate number is substantially a straight line with positive slope in relation to the X-axis (soda charge). Hence, where the cooking time is relatively short, as in the second stage of the process of the present invention, a given variation in soda charge due to uncontrollable factors (e. g. variations in moisture content of wood charged) will necessitate substantially less change in cooking time, to produce pulp of required permanganate number, than where the cooking time is relatively longer, as in the normal single stage process, operating on the same time cycle.

In Figure 1 of the accompanying drawings is shown a fiow sheet illustrating the normal singlestage cooking treatment for the production of pulp by the soda process, and in Figure 2 of the drawings is shown a flow sheet illustrating the two-stage process of this invention.

In Figure 2 the full lines indicate the first stage treatment and the dotted lines indicate the second-stage treatment.

Figure 3 is a graph showing quantity of caustic soda used in the treatment of wood against quantity neutralized and is referred to more fully later in this specification.

Example 1 The following is an example, for comparison purposes, of a normal single stage alkaline cooking treatment on Eucalyptus gigantea wood to produce a pulp having a permanganate number of about 15:

113 gms. of air dried wood (Eucalyptus gigantea) containing 100 gms. oven-dry wood was cooked with sufficient white liquor to give a soda charge of 24% NaOI-I on O. D. wood at a liquor ratio (i. e. mls. liquor/gms. O. D. wood) of 5.0 to 1. The temperature was raised to 170 C. over a period of 1.9 hours and the cook held at 170 C. for 1.28 hours. The permanganate number of the pulp was 14.9 and the concentration of caustic alkalinity in the black liquor was 20.6 gms. per litre (expressed as NaOH) Example 2 The following is an example of a two-stage cooking treatment according to this invention on the same type of wood to produce a pulp having approximately the same permanganate number (i. e. 15):

113 gms. of air-dry wood (Eucalyptus gigautea) containing 100.0 gms. oven-dry wood were charged to a digester with 500 mls. of black liquor obtained from a previous soda cook and containing 23.4 g. p. l. of caustic alkalinity expressed as 8. grams NaOH per litre. The cock was heated to 170 C. over a period of 1.9 hours and the digester held at this temperature for 0.25 hour. The digester was then shock-cooled by cold water and the liquor effluent from this impregnation treatment titrated potentiometrically to determine the amount of caustic alkalinity which had been neutralized. This liquor contained 7.6 g. p. l. caustic alkalinity. Thus the wood had neutralized 7.9 gms. caustic alkalinity expressed as equivalent NaOH.

The liquor was drained from the wood and white liquor added to the digester in sufficient quantity to provide 16.1% NaOH (i. e. 24.0 minus 7 .9) on the wood. This second stage was carried out by heating to 170 C. over a period of 1.9 hours and this temperature was held for 1.28 hours. The cook was shock-cooled in water and the pulp obtained had a permanganate number of 15.4. Th black liquor from this stage contained 22.5 g. p. l. of caustic alkalinity expressed as NaOI-I, and 310 mls. of black liquor were recovered. Two cooks were conducted under these conditions to provide sufiicient black liquor for the treatment described in Example 3.

Example 3 The black liquor from the second stage treatment of Example 2 was used to treat new wood in the following manner:

113 gms. of air-dry wood (Eucalyptus y gautea) containing gms. of O. D. Wood were treated with 500 mls. of black liquor from the second stage of the treatments described in Example 2 above. The liquor ratio was thus 5.1 to 1. After treatment at 170 C. for 0.25 hour and shockcooling, the concentration of caustic alkalinity in the liquor was 7.6 g. p. 1. (expressed as NaOH). Thus acidity in the wood equivalent to 7.4% NaOH on O. D. wood had been neutralized.

A second-stage treatment was then carried out with the addition of white liquor to the charge to give 16.6% NaOH (24.0 minus 7.4) on the O. D. wood. It was cooked at 170 C. for 1.28 hours, and the permanganate number of the pulp obtained was 15.6. The concentration of caustic alkalinity in the black liquor from this second stage was 22.9 g. p. l. (as NaOH).

Example 4 113 gms. of air-dry wood (Eucalyptus gigantea) containing 100 gms. of O. D. wood were treated with 500 mls. of black liquor obtained from a previous soda cook and containing 20.3 g. p. l. of caustic alkalinity expressed as grams NaOH per litre. The cock was heated to 170 C. over a period of 1.9 hours and the digester held at this temperature for 0.25 hour. The digester was then shock-cooled in water and the liquor from this impregnation treatment titrated potentiometrically to determine the amount of caustic alkalimty which had been neutralized. This liquor contained 5.66 g. p. 1. caustic alkalinity. Thus the Wood had neutralized 7.3 gms. caustic alkalinity expressed as equivalent NaOH.

The free liquor was drained from the wood and mls. of liquor remained entrained in the wood. The liquor charged to the second stage was made up as follows:

White liquor (111.4 g. p. l. NaOI-I) mls 150 Black liquor 20.3 g. p. l. free NaOI-I) mls 200 Thus the liquor ratio was 5:1 and 16.7% NaOH (24.0 minus 7.3) on O. D. wood was added as white liquor.

amass?" 'fE-his second zsWse'ttreatment was scan-ted out by heating to 17-0-: 'C.mverea: erie1l:di'i "ehoui s, and {this -temperature awa held for hours.

.1I1he:cookmesshock-coolen imwaterpa d the'tpulp [obtained :had r-a Ipermaxiganate number m we. 'lfIhebltick iiqu or from lthis'istage eontained 27%9 -tp. :4]- 50f ,caustimalkalini ty texpressed as' NaGH.

Example 5 who following is an example or the proc'ess of --Lthis invention :as tamilie'd to Wife 'i 'suhjhaite 'Ofikl'dft fnrecess.

Woodchatge'cl ldugmstOlDfEucaflyptus gigzmteh. :L vquQR-rfliniQwthat. Total talkali changetl Sex- 22%010. 0. .13. wood.

pnessejd as memo. D. Wood.

ofiulnhiditm 25% 1.25 hours. -1410 14.9.

was heated to 170 ('3. over a period of 1.9 hours and held at this temperature for 0.50 hour. The digester was then shock-cooled in water and the liquor drained from the wood. The liquor was titrated potentiometrieall and the amount of caustic alkalinity remaining in theliquo'r was n11. Thus the wood had neutralized 5.65% NaOI-I on the O. D. Wood.

The second stage was charged with White liquor containing 16.35% total alkali (NaOH and NazS) expressed as NaOI-I, and the sulphidity of this liquor was 25%. The liquor ratio was made up to 4.521 by the addition of water.

The digester was heated to 1 70 C. Wei" a period at 1.9: hours and held at this temperature for 1.25 hours; The; cook shock-cooled in cold water and the pulp obtained: had; a permanganate numbe: of 12 .0.. The black liquor eiiI-uent: from this stage contained caustic alkalinity equivalent to 131% g. p. I. expresseetas NaOili- The following is a plant scale example of the process, using soda.- cooking-1 Suflicientwood (mixed. species of Eucalyptus) was: hanged to filla: digsiierhavillg' a volumetric capacity (it 265 0 cutie feet. Theatriount of wood changed? Wasequivalent'tw ahout 303000 13 0s; 0. Di wood; Eli-Gilfigaliohs ofhlacli liiguorfi'om pt'evifo'us sodacooks ami'i containing: appi"oximatelIt pit 1 a caustic alkalihity; eX'preSSed aS'NaGH, was armed; 'Ehe digester was heatedltb 170"" C1 over 33 periodi of 2:0 hours by means of an external steam h-eate'n through whieltliquor; withdrawn from thehottbmoftlire= digeste'r; was passed and returnedtt e the tbgi ofi'the'wligesteri The masswas heldat lflfi cfvfo'r hour and' *tl ie'ftee liquor" was; then re-ieasezh Home the oti gester by way of: a vflilvfilldip'ip'fllihef; thewe-nttame tof'whieh:wa'sts it uatedsheneatt'isscre em plates iin thhottomfi of the cligester.

The 11(11101' leaving the digesteweam.

75" stage process, and the same pulp output 0. o'ver a eachef tie-h me; 11a was helii ttilis eium with the titintspiieie eat abbii-t 106 ficient white liquor was added to stif aoHmfiDfi-wo teal-1am tu reached, the senate number or "this invention with t e of the rni'ai iiigitp'r" s it may t a st in a 'eofnmereiai anel ieatioe er the p "me to nn the digest-er is 1 hiii if, th time to dtal'ii Uncle: these. conditions the comparative tifiit of a normal single=stae ook anti of the tea; stage process of this invention may be illustrated by the following table:

3 Single-stage mam-em Tithe to heat to maximum tempefatu r'e time: to ana recfiafge" Time to heat to maximum term if this" invention may lfecar thus be obtained by the process of this invention with substantial reduction in alkali consumption.

Thus, in a single stage cook using Eucalyptus gigantea, on an 8-hour cycle, the maximum cooking temperature being 170 C. and the pulp produced having a permanganate number of 15, the new alkali required was 18.6% NaOH on O. D. wood, whereas in the two-stage process of this invention as carried out in Examples 2 and 3 above, the new alkali consumption was 16.6% and 16.1% (NaOH on O. D. wood), respectively.

I have ascertained that the time required in the second stage of a two-stage cooking treatment according to this invention is substantially the same as is required in a, normal single stage: cook in which the soda charge is the same as the total soda charged over both stages in the two-stage treatment That is, if the time required at 170 C. to give pulp with permanganate number using 24% NaOH on the O. D. wood in a normal single stage cook is 1.28 hours, then in a two-stage cooking treatment according to this invention where 8% NaOH on the O. D. wood is neutralized in the first stage, the second stage using a soda charge of 16% NaOH and producing at 170 C. a pulp of permanganate number 15 will require 1.28 hours.

It will be clear to those skilled in the art that if the soda charge in the second stage of the process is increased, thereby reducing the cooking time, the black liquors from these stages will contain greater concentrations of unneutralized alkali, and that a point will be reached where the amount of soda available from these liquors willexceed that which can be neutralized in first stage treatments. If the soda charge in the second stage is reduced the cooking time required in the second stage is increased and the concentration of caustic alkalinity in the black liquor from this stage is reduced. A point will be reached where all the free alkali can be neutralized very easily in the first stage treatments, but this is offset by the longer cooking time required in the second stage. I have found that it is desirable that the amount of soda charged to the second stage should be controlled so that the concentration of caustic alkalinity remaining in the black liquor from this stage is below a figure which will permit the neutralization of at least 80% of this alkali in subsequent first stage treatments.

If the wood will neutralize at a maximum the equivalent of, say, 11% NaOH on the wood from black liquor in a first stage treatment, the optimum conditions under which to operate the process are those giving black liquor from the second stage containing a concentration of caustic alkalinity such that in normal operation approximately :c% NaOH on the Wood is charged to first stage treatments. This will then mean that the time of cooking in the second stage is at a minimum consistent with the neutralization of the maximum amount of caustic alkalinity in the first stage.

If the residual alkalinity in the black liquor from the second stage treatments is greater than 1% NaOH on the wood, the excess alkali unneutralized in the first stage will be carried through to the soda recovery system, thereby increasing recovery costs, while if the residual alkalinity is less than 50% the time of cooking in the second stages will be unnecessarily prolonged.

The control of the system to achieve this end may be carried out as follows.

The wood to be used is tested by treating samples containing known 0. D. weights of wood with increasing amounts of caustic soda over the range of, say, 6% to 14% as NaOH/O. D. wood, and with a fixed liquor ratio. The laboratory experiments are carried out by heating the reaction mixture at the same rate and. for the same time at maximum temperature as would be used in a normal first stage treatment under mill conditions. After shock cooling at the conclusion of the first stages the liquors from these experiments are analysed by potentiometric titration to determine the residual caustic alkalinity remaining in the liquor, and from these results the percentage of the soda which had not been neutralized is calculated. The curves obtained by plotting soda applied (X axis) against percentage of soda neutralized (Y axis) are of the form shown in Figure 3. From these curves it can be seen that, with increasing soda charge, the percentage of the charge neutralized decreases progressively. For the purposes of this work we regard a minimum of neutralization as being desirable, and on this basis would regard the soda charge at which 80% of the soda is neutralized (i. e. R% NaOH on OJD. wood) as being the reatest amount that should be added in first stage treatments with the wood under consideration. Hence R equalsalkali charge (as per cent NaOH on O. D. wood) at which 80% of the alkali is neutralized by the wood in a first stage treatment.

In the fiowsheet shown in Figure 2, it can be seen that black liquor leaving the digester with the pulp after the second stage will suffer some dilution during pulp washin operations. The amount of dilution occurring in any one mill is reasonably constant. The dilution occurring may be represented by P, that is,.P equals the con centration of caustic alkalinity in the black liquor leaving the second stage divided by the concentration of caustic alkalinity in the liquor after dilution. If the volume of black liquor charged to first stage treatments is "Q gallons per lbs. of O. D. wood, and the maximum amount of caustic alkalinity to be charged in this stage is R% NaOH/0. D. wood (determined as described above) then the caustic alkalinity in the black liquor charged should not exceed R/Q lbs. of NaOH per gallon, i. e. IGOR/Q lbs. NaOH per 100 gallons. Thus, the conditions of cooking must be controlled by increasing or decreasing the soda charge in the second stage so that the concentration of caustic alkalinity in the black liquor at the completion of this stage does not exceed (IGOR/Q) P lbs. NaOH per 100 gallons.

The concentration of caustic alkalinity at the completion of the second stage increases linearly with increases in the soda charge used in the second stage if the pulp produced has thesame permanganate number and the amount of soda absorbed in the first stage is constant. Hence with increasing soda charge in the second stage, the time of cooking required is decreased and the caustic alkalinity in the black liquor from such treatment increases.- The optimum conditions for two stage cooking will thus vary with different woods, but are such that the second stage cooking time is as short as possible, the lower limit being determined by the above mentioned preferred limitation on the caustic alkalinity of the black liquor eflluent from the second. stage,

13 Example 7 EXPLANATION OF APPLICATION OF FORMULA IN EXAMPLE 6 The maximum quantity of caustic soda which can be added as white liquor in the second stage of the process described in Example 6 is calculated from the above mentioned formula as follows:

For the neutralization of 80% of the alkali in the first stage treatment an alkali charge of about NaOH 0. D. wood (see Figure 3 Graph B can be used. The term R of the formula is therefore 10.

The volume of black liquor charged to the first stage treatment was 9000 gallons and the amount of wood charged was equivalent to 30,000 lbs. 0. D. wood. The term Q (gallons black liquor per 100 lbs. 0. D. wood) is therefore 30.

The dilution factor P (concentration of caustic alkalinity in the black liquor leaving the second stage divided by the concentration of caustic alkalinity in the liquor after dilution) equals or 1.7.

Hence the caustic alkalinity of the black liquor discharged from the second stage treatment should contain less than RP/Q lbs. NaOH per gallon, i. e. .563 lb. NaOI-I per gallon, or 56.3 lbs. NaOH per 100 gallons.

Now, the quantity of liquor present in the wood at the conclusion of the first stage was 4500 gallons, and the quantity of liquor to be added to the second stage was 7000 gallons making a total of 11,500 gallons. Of this 1800 gallons was evaporated during blowing of the digester, leaving 9700 gallons. This liquor must contain less than 56.3 lbs. NaOH per 100 gallons, that is, less than 5540 lbs. NaOH.

The residual liquor in the wood after the first stage (4500 gallons) contained 3.0 g. p. l. as NaOH, i. e. a total of 135 lbs. NaOH. The black liquor make-up added for the second stage contained 23 g. p. l., and the white liquor 120 g. p. l.

If as equals the maximum volume of white liquor chargeable to the second stage, in gallons, the maximum quantity of NaOH (in lbs.) to be charged to the second stage will be Of this 8% NaOI-l is to be neutralized by the wood in the second stage, i. e. 8% of 30,000 lbs. or 2400 lbs. Hence the maximum residual free NaOH (in lbs.) to be present in the liquor from the second stage is (7000a:) 120a: 135+23- 100 2400 By the formula, this maximum quantity is 5540 lbs. Hence i. e. 11::6328 gallons.

The quantity of NaOH to be added as white liquor for the second stage treatment must therefore be less than D. wood As the example shows, the amount of alkali added was 16% NaOH/O. D. wood and the con- 14 centration of free NaOH in the black liquor was 23 g. p. 1., both of which results are less than the maximum allowed, so that not less than of the soda charged to the first stage will be neutralized in subsequent cooks where this method of control is exercised.

I claim:

A two-stage alkaline pulping process for the treatment of non-coniferous woods for the production of paper pulp consisting essentially in introducing a charge of wood chips into a digester, adding to the digester black liquor from a previous second stage cooking treatment in such quantity as to provide between 30 and 50 gallons per pounds oven-dry wood, the alkalinity of the black liquor being equivalent to between 15 and 35 grams of active caustic soda per liter, heating the charge to a temperature between C. and C. with the digester closed except when gassing to remove non-condensable gases, maintaining the charge in such temperature range for a period between 6 and 30 minutes sufiicient to substantially neutralize the natural acidity of the wood while neutralizing 80% to 100% of the caustic alkalinity of said black liquor, discharging the substantially exhausted liquor from the digester, then without any intervening treatment of the wood adding to the digester a digestion liquor consisting of white liquor and black liquor, the quantity and alkali concentration of the white liquor being such as to provide an effective alkali charge (expressed as NaOH) of 15 to 20 pounds per 100 pounds oven-dry wood and the total quantity of the digestion liquor added being between 15 and 35 gallons per 100 pounds oven-dry wood, original charge, heating the charge to a temperature between 160 C. and 175 C. with the digester closed except when gassing to remove non-condensable gases, maintaining the charge at a temperature in said range for a period of one to five hours, discharging the pulp and liquor from the digester, and recovering the black liquor for use in a subsequent firststage treatment.

ARTHUR RAYMOND SLOMAN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,690,954 Spencer Nov. 6, 1928 1,882,391 Morterud Oct. 11, 1932 1,949,669 Wells Mar. 6, 1934 2,047,314 Dreyfus July 14, 1936 2,101,253 Gunther et a1. Dec. 7, 1937 2,190,193 Richter Feb. 13,, 1940 2,247,204 Schorger et al June 24, 1941 2,292,390 Meiler Aug. 11, 1942 2,466,290 Wells Apr. 5, 1949 OTHER REFERENCES Wells: Article in Paper Trade Journal, Nov. 9,, 1944, pp. 30-36 (page 31 pertinent).

Wells: Article in Paper Trade Journal, Oct. 30, 1947, pp. 42-47 (page 43 pertinent).

Davis: Article in Technical Association Papers, Series XVII, Sept. 1933, pp. 330-343 (pp. 336-7 pertinent).

Wood Chemistry by Wise, published by Reinhold Publishing Corp., New York, p. 716 (1944). 

1. A TWO-STAGE ALKALINE PULPING PROCESS FOR THE TREATMENT OF NON-CONIFEROUS WOODS FOR THE PRODUCTION OF PAPER PULP CONSISTING ESSENTIALLY IN INTRODUCING A CHARGE OF WOOD CHIPS INTO A DIGESTER, ADDING TO THE DIGESTER BLACK LIQUOR FROM A PREVIOUS SECOND STAGE COOKING TREATMENT IN SUCH QUANTITY AS TO PROVIDE BETWEEN 30 AND 50 GALLONS PER 100 POUNDS OVER DRY WOOD, THE ALKALINITY OF THE BLACK LIQUOR BEING EQUIVALENT TO BETWEEN 15AND 35 GRAMS OF ACTIVE CAUSTIC SODA PER LITER. HEATING THE CHARGE TO A TEMPERATURE BETWEEN 160 AND 175* C. WITH THE DIGESTER CLOSED EXCEPT WHEN GASSING TO REMOVE NON-CONDENSABLE. GASES, MAINTAINING THE CHARGE IN SUCH TEMPERATURE RANGE FOR A PERIOD BETWEEN 6 AND 30 MINUTES SUFFICIENT TO SUBSTANIALLY NEUTRALIZING THE NATURAL AACIDITY OF THE WOOD WHILE NEUTRALIZING 80% TO 100% OF THE CAUSTIC ALKALIINITY OF SAID BLACK LIQUOR DISCHARGING THE SUBSTANTIALLY EXHAUSTED LIQUOR FROM THE DIGESTER, THEN WITHOUT ANY INTERVENING TREATMENT OF THE WOOD ADDING TO THE DIGESTER A DIGESTION LIQUOR CONSISTING OF WHITE LIQUOR AND BLACK LIQUOR , THE QUANTITY AND ALKALI CONCENTRATION OF THE WHITE LIQUOR BEING SUCH AS TO PROVIDE AN EFFECTIVE ALKALI CHARGE (EXPRESSED AS NAOH) OF 15 TO 20 POUNDS PER 100 POUNDS VEN-DRY WOOD AND THE TOTAL QUANTITY OF THE DIGESTION LIQUOR ADDED BEING BETWEEN 15 AND 35 GALLONS PER 100 POUNDS OVEN-DRY WOOD, ORIGINAL CHARGE, HEATING THE CHARGE TO A TEMPERATURE BETWEEN 160* C. AND 175* C. WITH THE DIGESTER CLOSED EXCEPT WHEN GASSING TO REMOVE NON-CONDENSABLE GASES, MAINTAINING THE CHARGE TO A TEMPERATURE IN SAID RANGE FOR A PERIOD OF ONE TO FIVE HOURS, DISCHARGING THE PULP AND LIQUOR FROM THE DIGESTER, AND RECOVERING THE BLACK LIQUOR FOR USE IN A SUBSEQUENT FIRSTSTAGE TREATMENT. 